1. Field of the Invention
This invention relates to electrochemical cells for reacting a liquid electrolyte with a gas, each ell unit having a porous, self-draining electrode.
2. Description of the Prior Art
Packed bed electrolytic cells are known for Oloman et al U.S. Pat. No. 3,969,201 and U.S. Pat. No. 4,118,305. Improvements in these cells have been disclosed by McIntyre et al in U.S. Pat. No. 4,406,758; U.S. Pat. No. 4,431,494; U.S. Pat. No. 4,445,986; U.S. Pat. No. 4,511,441; and U.S. Pat. No. 4,457,953. These packed bed electrolytic cells are particularly useful for the production of alkaline solutions of hydrogen peroxide.
Among known bleaching agents, hydrogen peroxide is at the present time being increasingly used, in particular for bleaching or brightening materials such as textiles or paper pulp. Hydrogen peroxide has the great advantage over other bleaching agents, in particular chlorine and its compounds, in that because of its mild action, it attacks the fibers of the material to be treated to a much lesser extent and gives a better finish.
Hydrogen peroxide is generally used in bleaching in the form of a stabilized alkaline solution of low peroxide concentration, the weight ratio of alkali, i.e., NaOH to hydrogen peroxide (H.sub.2 O.sub.2) generally being about 0.5 to about 2.0. The action of hydrogen peroxide in bleaching consists essentially of destroying or decolourising the natural dyes by oxidation, or by rendering them soluble. Even though the mechanism of these reactions has been little studied, it is generally assumed that the perhydroxyl ion, HOO.sup.- is responsible for the bleaching.
Present-day bleaching solutions based on hydrogen peroxide have the great disadvantage with respect to other conventional bleaching solutions (in particular hypochlorite-based solutions) of being relatively costly. Their widespread use is very dependent upon economic considerations, particularly when large quantities of low-value material such as paper pulp are to be treated. Present-day bleaching solutions are nearly always prepared by simple dissolving and dilution, starting from commercially available chemicals. Commercially available hydrogen peroxide is a particularly costly substance, as it is manufactured only in a small number of large industrial plants, and it has therefore to be highly concentrated for storage and transport purposes before being distributed. At the present time there is a need to replace the highly concentrated, commercially available hydrogen peroxide by on site manufacturing methods which enable dilute solutions of hydrogen peroxide to be produced directly, in order to reduce bleaching costs. However, up to the present time no economically satisfactory method has appeared.
Hydrogen peroxide is used not only for bleaching purposes, but also in an increasing number of other processes, in particular in the pollution control field. However, treatment solutions used for this purpose are likewise almost always prepared from highly concentrated hydrogen peroxide with the same disadvantages as heretofore stated.
In Grangaard, U.S. Pat. No. 3,607,687; U.S. Pat. No. 3,462,351; U.S. 3,507,769; and U.S. Pat. No. 3,592,749 there are disclosed electrolytic cells for the production of hydrogen peroxide in which the peroxide is produced in the cathode compartment of the cell which contains a cathode depolarized utilizing an oxygen containing gas. The electrochemical cells of Oloman et al and McIntyre et al disclosed in the patents cited above, are improvements over the cells of Grangaard partly as the result of the use of the novel electrode material disclosed in U.S. Pat. No. 4,457,953 in which there is disclosed a method for the production of coated particles for use in a packed bed electrode electrochemical cell.